Aims/hypothesis: We examined the associations between prepregnancy dietary patterns and the incidence of gestational diabetes mellitus (GDM) in a population-based cohort study of women of reproductive age.Methods: The Australian Longitudinal Study on Women’s Health included 3,853 women without pre-existing diabetes who were followed-up between 2003 and 2012. Prepregnancy dietary patterns were derived using factor analysis based on 101 food items from a validated food frequency questionnaire. GDM was self-reported and validated in a subsample. Multivariable regression models with generalised estimating equations were used to estimate RR and 95% CI.Results: During 9 years follow-up, 292 GDM cases (4.4%) were documented in 6,626 pregnancies. No associations were found for the ‘Fruit and low-fat dairy’ and ‘Cooked vegetables’ patterns. The ‘Meats, snacks and sweets’ pattern was associated with higher GDM risk after adjustment for socioeconomic, reproductive and lifestyle factors (RR [95% CI] per SD increase in score: 1.38 [1.02, 1.86]). Further adjustment for BMI attenuated the results (1.35 [0.98, 1.81]). In stratified analysis, the ‘Meats, snacks and sweets’ pattern was associated with significantly higher GDM risk in parous and obese women, and in women with lower educational qualifications. The ‘Mediterranean-style’ pattern was associated with lower GDM risk in the fully adjusted model (0.85 [0.76, 0.98]).Conclusions/interpretation: These findings support general dietary recommendations for women of reproductive age to consume a diet rich in vegetables, whole grains, nuts and fish, and low in red and processed meats and snacks. Further prospective studies are needed to confirm these findings.